The sampling of 192 stations in the same basin of the Central Pyrenees — the river Neste d'Aure and its tributaries — between 2 500 and 500 meters altitude, has made possible the ecological study of 108 species of Trichoptera.

I. — The general conditions (topography, hydrology, geology, climate) make the valley of Aure a good example of a valley of the Central Pyrenees. For each of the tributaries studied are given: topography, hydrography, nature of the soil, vegetation and human influences. Relative data in each station are listed in an annex: reference, localisation, degree of investigation (samples, number of species and of larvae taken), physico-chemical properties.

II. — Benthic samples have been taken with a hand-net from surfaces varying between 0,20 and 0,30 m2. A correction factor (x 2) was introduced so that moss samples might be comparable to samples from stony substrata.

The representation of the fauna from each station groups all the samples taken into different habitats. The symbol to denote abundance used for each species shows the greatest number of larvae taken in one of the samplings of the station (fig. 5).

14 species have been classed as fundamental according to their frequency in the higher classes of abundance. Their coenotical affinities, established by the method of the mean square of contingency, made possible the determination of different groups of Trichoptera in the whole of the valley of Aure (fig. 8 to 21).

Each group is characterized by its specific components and its ecological components. Starting from typical stations where the group is well represented, different tendencies appear corresponding to interferences or to contact zones between the groups. The communities of the different habitats are separated in the typical stations mainly with regard to the speed of the current and the nature of the substratum. Comparison of the groups shows:

— the combined influence of altitude and of range of temperature in the spatial distribution of the groups (fig. 24),

— the importance of the intermediate groups (fig. 25).

Two types of populations can be distinguished in the water courses of the valley of Aure. The first one includes stenothermic species from high altitudes; it is found in the springs and the cold torrents of the Massif de Néouvielle. The second is composed of eurythermic species; starting from the streams in low and medium altitudes it spreads to the outflows of lakes in high mountains. These two types are mixed in the intermediate stations:

— Outflows of cold bogs and streamlets in the subalpine zone in high mountain,

— medium altitude regions of streams which descend rapidly from high mountain in a steep slope,

— regions of the springs of streams at medium altitudes.

The topographical particularities of the valley of Aure do not allow the development of certain mid-altitude groups.

The localisation of Trichoptera in the different habitats of running water was studied. The abundant species in a station tend to spread into the different habitats of that station. The larvae of several species which live in rapid currents in the stations with high maximum temperatures make up a more important part of the communities in slow currents or in pools when the maximum temperatures drop. For Trichoptera, the dispersion of percentages (σ) is high in mosses and in pools. On stony substrata the values of σ are similar in springzones at different current speeds. In the mountain streams (rhitron) , the values of σ diminish accordingly as slow currents become faster. These conditons are inverted in the last station of the river Neste, where it is influenced by the proximity of the Garonne (potamon).

In conclusion, stress is laid on the transitor y character of the benthic biocoenoses and on the importance of range of temperature in the zonation of running waters.

III. — Biocoenotic analysis of the species found in high mountains shows three series of affinity-clusters (tableau XVII). These clusters are such that one merges into the next in a given series. Their variations are correlated with two main factors: range of temperature and speed of the current (fig. 29). The influence of daily variations of temperature on the fauna of three running water stations is indicated for three streams of which the range of temperatures are different. The role of the intrinsec factors was studied in the distribution of five morphological types and in the behaviour of the adults. The localisation of the affinity clusters is shown in two typical streamlets at a high altitude differing essentially in their orientation and their origin.

IV. — The growth of several species at high and medium altitudes was studied. Larval instars were determined by measurements of the head of the larvae (fig. 34 A).

— The growth of Rhyacophila occidentalis occurs mainly in autumn and in winter; the complete cycle lasts one year (fig. 35). The young larvae can be found in pools or slow running water up to the second and third instars; after that they prefer rapid currents (fig. 34 B). These changes coincide with the change to a carnivorous diet in the third instar.

— According to altitudes the cycle of Rhyacophila evoluta can last one, two or three years (fig. 36 B, C, D). Growth in winter is possible at a low altitude (fig. 36 A). In high altitudes one can distinguish two or three breaks in development each winter in the egg stages, the third, fourth and fifth instars (fig. 37 A, B, C).

— Rhyacophila angelieri and Rhyacophila contracta have a cycle which is identical to that of Rh. evoluta: two years in station 5.4 (Rh. angelieri and Rh. contracta); three years in station 5.1. (Rh. contracta).

— Rhyacophila mocsaryi is a species found in medium altitudes with a mainly summer growth. The adults appear very early in the year (fig. 38).

— The cycle of Micrasema morosum lasts two years in high altitudes. Two breaks in development occur, one in each winter — in the fourth instar and the nymphal stage (fig. 39 A). The histograms for a mid-altitude population indicate the possiblility of a one or two year cycle (fig. 39 B).

— Micrasema vestitum completes its cycle in one year in stations 7.2 and 7.3. There is a slight retardation in the population of the coldest station (fig. 40).

— The cycle of Drusus rectus lasts two years at a high altitude (fig. 41 A) and one year at a medium altitude (fig. 41 B).

— For Allogamus auricollis, the differences in size between two populations at high and low altitudes were studied for each larval instar (fig. 42 B) and for the adults (fig. 42 A). The differences of size between the low and high altitude populations is accentuated in fourth and fifth instar. From the dissection of several $, it would seem that the maturation of the ovaries is more rapid at a high altitude (fig. 43). The development cycle lasts one year at high and low altitudes but the rhytmus of growth are different (fig. 44).

— The development of Limnephilus bipunclatus follows the evolution of temporary ponds (fig. 45). The break in development at the egg stage corresponds to the drought. Evolution varies according to the date when the drought ceases (in autumn or in spring).

At high altitudes, the period of snow cover corresponds to alternations between dormancy and sluggish activity. The Trichoptera at high altitudes are characterized by the ability to remain in a state of in activity at any instar in their cycle. The break in the fifth instar and in the nymphal stage at high temperatures brings about a levelling of low and high altitude populations in species of the autumn fauna: the appearance of the adults is not retarded by altitude.

The seasonnal evolution of the fauna was studied in two stations at a high and a medium altitude. At the higher station, in winter, there is a break in development, in different instars according to species. The growth of the larvae occurs mainly in summer and in autumn. Each year, after the severe dry season in winter, the floods from the melting snows cause a new distribution of the habitats and of the fauna. The development of the larvae lasts one or two years (fig. 46). The rate of flow in station 7.3 is more regular: the winter dry season is less severe, the spring floods are weakened by artificial barriers. The substrata eaters are very numerous on the stones in slow or moderate current; Odontocerum albicorne and Sericostoma personatum are found in muddy places. The fauna of the stones in rapid to very rapid currents is more heterogeneous. The development cycles of the larvae lasts one year (fig. 46).

The differences in the general conditions between the two stations can ben discussed with regard to their influence on the seasonal evolution of the fauna.

In conclusion the importance of behaviour and of specific reactions on the distribution of the Tricfoptera of the valley of Aure is emphasized.

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